Chemotaxis, a type of cell migration, is a highly integrated, multistep process that orchestrates embryonic morphogenesis, tissue repair and regeneration. The initial response of a cell to a migration-promoting agent is to polarize and extend protrusions in the direction of the attractant; these protrusions can consist of large, broad lamellipodia or spike-like filopodia that are driven by actin polymerization and can be stabilized by extracellular matrix (ECM) adhesion or cell-cell interactions. The CytoSelect™ Cell Migration Assay Kit utilizes polycarbonate membrane inserts (8 µm pore size) to assay the migratory properties of cells. The kit contains twelve 5 um polycarbonate membrane inserts in a 24-well plate, through which migratory cells are able to pass. Cells are quantified using the provided fluorometric dye. The 5 µm pore size is optimal for measuring the migration of monocytes and macrophages.